Duke is one of the oldest and most experienced engines of the Narrow Gauge Railway. He is also the mentor of Peter Sam and Sir Handel, from their older days on the Mid-Sodor Railway, when they were called Stuart and Falcon respectively. Although once lost for many years in an old shed that nature tucked away and forgot, after the Mid-Sodor Railway came to an end, he was found by rescuers, although quite by accident, when one came in through his shed roof. He may have a head teacher attitude and maintain his authority at all times, but can be quick to apply his knowledge and years of experience to help guide and correct any younger engine. He is proud of his name and keen to uphold the respect it merits. He can often be heard to say "That would never suit his Grace!" upon hearing or seeing something that he takes a disapproval to, or pedantically stating how he thinks someone would have adhered to the rules and regulations in his young days. Despite their teasing ways, Sir Handel and Peter Sam in particular have grown to become very fond of Duke and dearly nicknamed him "Granpuff".
Duke is a 0-4-0ST+T based on 'Prince' of the Ffestiniog Railway at Porthmadog, and the oldest working engine in the United Kingdom, being built by George England in 1863